Wireless communication devices usually comprise wireless receivers and transmitters. A down-conversion mixer is an important building block in a wireless receiver, which converts a high Radio Frequency (RF) fRF signal to an Intermediate Frequency (IF) fIF signal by virtue of a Local Oscillator (LO) fLO signal. The output of the mixer is an IF signal, and its frequency is given by fIF=|fRF−nfLO| where n is an integer.
If n=1, the IF signal is obtained by mixing the RF signal with the 1st harmonic of the LO signal, such kind of mixer is called a fundamental mixer; if n≠1, the IF signal is obtained by mixing the RF signal with the n-th order harmonic of the LO signal, such kind of mixer is called a xn sub-harmonic mixer.
The sub-harmonic mixer is driven by a low frequency LO signal. For example, an x2 sub-harmonic mixer requires an LO signal having a frequency of a half of that for a fundamental mixer. If a fundamental mixer is used for a receiver operating at a RF frequency above 140 GHz, a signal source with a frequency around 140 GHz is needed. Unfortunately, such a signal source is not commercially available. One solution is using a frequency multiplier together with a low frequency signal source. Another solution is using a sub-harmonic mixer.
Furthermore, the larger the number n is, i.e. the n-th order harmonic of the LO signal, the lower the LO signal frequency is. A low frequency signal source usually has a better phase noise performance than that of a high frequency one, because the quality factor Q of the resonator increases as the frequency decreases. Consequently, a high-order (n≧4) sub-harmonic mixer is desired, especially, in the case that the RF frequency is larger than 140 GHz.
In ELLIS, Thomas J. et al., A Planar Circuit Design for High Order Sub-harmonic Mixers, Microwave Symposium Digest, IEEE MTT-S International, 1997, an x5 sub-harmonic mixer with a single diode is disclosed. This x5 sub-harmonic mixer has a conversion loss of 15 dB with an LO signal power of 12 dBm.
In KANAYA, K. et al., A 94 GHz High Performance Quadruple Subharmonic Mixer MMIC, Microwave Symposium Digest, IEEE MTT-S International, 2002, a sub-harmonic mixer with an anti-parallel diode pair is designed and characterized. It has a conversion loss of 9.2 dB with an LO signal power of 5 dBm, when operating with the 2nd order harmonic of the LO signal, and it has a conversion loss of 11 dB with an LO signal power of 10 dBm, when operating with the 4th order harmonic of the LO signal.
U.S. Pat. No. 4,320,536 discloses a sub-harmonic pumped mixer circuit utilizes quarter wavelength transmission lines at an LO frequency to provide high RF to LO port isolation while allowing the RF and LO ports, as well as an IF port, to be connected directly to a semiconductor switch. A transmission line suppressor network connected between the semiconductor switch and the RF port allows operation at any selected even LO harmonic by suppressing response to even harmonics of the LO frequency below the selected harmonic and includes transmission line suppressors which are each a quarter wavelength at the frequency to be suppressed.
In LIU, Zhiyang et al., High-Order Subharmonically Pumped Mixers Using Phased Local Oscillators, IEEE Trans. Microw. Theory Tech. July 2006, vol. 54, no. 7, pp. 2977-2982, an x8 sub-harmonic based on two anti-parallel diode pairs is disclosed, which has a conversion loss of 8 dB with an LO signal power of 12.5 dBm.
Other kinds of sub-harmonic mixers, as shown in FIG. 1, are Gilbert type mixers, where emitter-coupled pairs driven by differential LO signals are either in parallel or stacked, to enhance the 2nd LO harmonic and suppress the 1st harmonic. For example, in TSAI, J.-H. et al., 35-65 GHz CMOS Broadband Modulator and Demodulator with Sub-harmonic Pumping for MMW Wireless Gigabit Applications, IEEE Trans. Microw. Theory Tech., October 2007, vol. 55, no. 10, pp. 2075-2085, an x2 sub-harmonic mixer (Gilbert type) is disclosed and has a conversion loss about 6 dB with an LO signal power of 7 dBm; in SHENG, L., et al., A Wide-bandwidth Si/SiGe HBT Direct Conversion Sub-harmonic mixer/Downconverter, IEEE Journal of Solid-State Circuits, September 2000, vol. 35, no. 9, pp. 1329-1337, an x2 sub-harmonic mixer is described and it achieves a conversion gain of 12.6 dB with an LO signal power of 10 dBm; and in U.S. Pat. No. 6,810,242, a sub-harmonic is disclosed which uses two stacks of switching cores with high order symmetry to reduce unwanted harmonic generation and uses Complementary Metal-Oxide Semiconductor (CMOS) transistors to improve headroom.
Another kind of sub-harmonic mixer is shown in FIG. 2, where the LO signal is applied at the gate/base of the transistor and the RF signal is applied at either the drain or the base. Such kind of mixer is called as “gate mixer”. In GUNNARSSON, Sten E., Analysis and Design of a Novel 4 Subharmonically Pumped Resistive HEMT Mixer, IEEE Trans. Microw. Theory Tech., April 2008, vol. 56, no. 4, an x4 sub-harmonic mixer is shown, which has a conversion loss of 15 dB with an LO signal power of 8 dBm. In MAO, Yanfei, et al., 245-GHz LNA, Mixer, and Subharmonic Receiver in SiGe Technology, IEEE Trans. Microw. Theory Tech., 2012, vol. 60, no. 12, pp. 3823-3833, an x4 sub-harmonic mixer is described and it has a conversion loss of 5 dB.
In WINKLER, S. A. et al., Ultra-high-order Self-oscillating Mixers Based on Multiple-element Technique, Electronics Letters, December 2009, vol, 45, no. 25, an x6 sub-harmonic mixer integrated with an oscillator (self-oscillating mixer) is demonstrated. The mixer achieves a conversion gain of 2.8 dB.
Finally, in NING, Xiaoxi, et al., A 79 GHz Sub-harmonic Mixer Design Using a 1 um InP DHBT Technology, International Conference on Microwave and Millimeter Wave Technology (ICMMT), 2012, a sub-harmonic mixer is described which has a frequency multiplier, where the LO signal is applied at an input of a frequency doubler. At the output port of the frequency doubler, the 1st order of the LO signal is suppressed by a quarter wavelength open stab.
The sub-harmonic mixer utilizing diodes as described above has a conversion loss and needs an LO signal power larger than 5 dBm. Even though a Gilbert type sub-harmonic mixer has potential to get a positive conversion gain, the Gilbert type sub-harmonic mixers, as well as most of the gate mixers need an LO power divider and phase shifter which is built by either transmission lines or lumped components such as resistors and capacitors. The power divider/phase shifter has a certain frequency bandwidth, which limits the frequency bandwidth of the mixer, and furthermore, the losses of the power divider/phase shifter increases the LO power consumption. If a frequency multiplier is utilized in the sub-harmonic mixer, it will cause extra power consumption.